Vane pumps are used very frequently especially for the supply of pressure oil to auxiliary power steering systems. For regulation of pressure flow for auxiliary power steering, it has been known to restrict pump flow pressure passage adjacent the pump outlet which increases the flow speed and thus decreases the static pressure on one side, viz., the rear side of the flow regulating piston. Accordingly, pressure on one surface of a regulator piston becomes relatively greater i.e., on the upstream face directly exposed to outlet chamber pressure and the flow regulating piston experiences an opening force due to differential pressuer, depending on pump speed. Such piston movement opens a passage from pump outlet to pump inlet increasing the pump circulation between outlet and inlet and decreasing flow to a consumer device, e.g., a power steering system. Thus, on a graph of pump discharge (ordinate) plotted against speed (abscissa) a dropping characteristic line occurs, i.e., the pump discharge is not increased corresponding to the speed increase at the beginning of regulation, but is reduced.
Separation of characteristic lines are desirable for various reasons. Thus, for example, a decreasing characteristic line for auxiliary power steering at high rpms results in a better steering function. Moreover, the circulating pressure flow in the steering system decreases with a dropping characteristic for flow with rising rpms. The consequence is a lesser working effort of the pump which effects a lower operation temperature.
In the German OS No. 22 65 097 (U.S. Pat. No. 3,728,046) and the German OS No. 26 52 707, it has been known in the regulation of the pump flow to provide a flow regulating piston with a tapered conical flow control pin movably projecting into a housing restrictor bore. The varying diameter of the pin during movement of the flow regulating piston, effects an annular restrictor toroidal flow passage variable in cross section resulting from pressure actuation of the flow regulating piston. The course of the characteristic curve of flow after the beginning of regulation may be influenced thus by the profile of the regulating pin. In such construction, there is a drawback in that the conical pin used for the variation of the toroidal flow area does not always effect a precise course of the characteristic line. Differences in mass production is relatively great. Thus, for example, the pin may be disposed eccentrically in the restrictor bore and in certain circumstances may even contact the bore wall. As a result, movement of the flow regulating piston may be stopped, which in practice has happened.
Moreover, it is very difficult in use of a tapered pin to adjust a desired flow cross section for a certain actuated position of the flow regulating piston by reason of finishing techniques. As a result the governing point at which flow regulation occurs is determined imprecisely.
Another drawback of the prior art is that the tapering pin has a small diameter, especially at the control position for high volume flow. Thus precise production is expensive and such pins may easily break. A further disadvantage of the prior constructions in that flow regulating pistons and pressure relief bores or restrictor bores are aligned coaxially and thus the dimensions of the pumps are correspondingly larger.
Further, the separation of the courses of the characteristic curves for variable pressure conditions and rpms are limited.
For saving power and for avoiding high temperatures, it is desirable especially at high rpms and low pressures in the system, for example, for straight ahead travel at high speeds, to have only a slight oil circulation. However, in a steering movement, i.e., with a high operating pressure, a high flow is required to the power cylinder.
In the German OS No. 24 02 017 (U.S. Pat. No. 3,989,414), a rotary piston pump has been known in which the flow regulating valve with a cylindrically shaped restrictor insert is provided with a restrictor bore. In such construction the separation of the characteristic curves are likewise limited.